This invention relates to rapid restrike circuits for metal halide lamps, and more particularly, to a spark gap device for rapid restrike circuits having an improved electrode arrangement that increases its reliability and operational life.
Standard metal halide lamps having an arc tube disposed therein and containing constituents, typically require a relatively long duration such as 10 to 15 minutes, in which the power may be reapplied to the arc tube to initiate or restrike its arc condition following any power interruption to the lamp which cause the extinction of the arc condition of such a previously operated lamp. Such metal halide lamps suffer a disadvantage in certain lighting systems such as those employed in sports stadiums. For example, if the lighting system of the sports stadium experiences a power outage, it is desired that the initiation or restrike of the arc tube be attempted within a few seconds, rather than 10 to 15 minutes, so that the sporting activity being performed can be resumed within a relatively short time and observed by those people at the stadium and also by those people that may be viewing the event on television.
Metal halide lamps that allow for this restrike condition require a starting or ignition signal of a relatively high voltage and at a relatively high current level from external circuitry in order to initiate a desired ionization condition for the constituents within the arc tube. A related metal halide lamp and associated external circuit are described in U.S. Pat. No. 4,723,097 which is herein incorporated by reference. To accommodate the starting or restrike conditions needed for the related metal halide lamps, the external circuitry commonly employs a spark gap device such as a commonly known spark plug having relatively high voltage and current carrying capabilities. The spark plug has electrodes in which the ends are separated from each other by a predetermined gap to allow a spark to be conducted therebetween during its conductive state. Over a period of operational time of the spark plug, the ends of the electrodes erode, the separation gap becomes larger and the voltage potential necessary to be applied across the electrodes to render the spark plug conductive all increase at a significant rate until the spark plug is rendered inoperative which, in turn, renders the rapid restrike circuit inoperative.
In order to avoid this inoperative state of the rapid restrike circuit, a planned preventive maintenance plan is commonly implemented in which the spark plug is replaced on a periodic basis. It is desired that a more reliable spark gap device having an increased operational life be provided so as to correspondingly increase the reliability of the rapid restrike circuit.
Accordingly, it is an object of the present invention to provide a reliable spark gap device so that the reliability of the rapid restrike circuit used for metal halide lamps may be correspondingly improved.